The present invention relates to a scanning method and a scanning circuit and, more particularly, to a scanning method and a scanning circuit which use a display element of a liquid crystal or the like and are suitable for an active matrix type display having a driver built therein.
The so-called "active matrix display", which is formed on a substrate of glass or the like with switching elements such as thin film active elements, e.g., diodes or thin film transistors (which will be referred to as the "TFTs" for brevity) and which are combined with a substance having an electro-optical effect such as a liquid crystal, is featured by capability of forming a large-area, high-fineness and high-quality display. In addition, the display using the TFTs constitutes a driver of the TFTs so that it forms on the glass substrate not only a display unit but also a circuit for driving the display unit to reduce the number of connecting lines from the outside and the number of external drivers. This makes it possible to drop the production cost and to prevent the reliability from dropping due to inferior connection. Thus, many displays having the driver built therein are proposed in Japanese Patent Laid-Opens Nos. 56 - 92573 and 57 - 100467 and so on since they have been proposed in Proceedings of IEEE, 59, P1566 (1971). These circuit structures can constitute a signal circuit for generating a signal voltage to be applied to the wiring at a signal (or data) side, of a smaller number of TFT elements per line but still has room for improvements in the following points. First of all, the voltage applied to the signal electrode (or data line) of the display unit has its signal voltage applied to the signal electrode through a TFT element at the output step of a driver, when the TFT element is on. When the TFT element is then turned off, the voltage is held by the capacitor Cl of the signal electrode. These operations are accomplished for a period, in which one of the scanning lines is selected so that a scanning voltage for turning on the TFT element of the display unit is applied to the scanning electrode. This makes it necessary for the voltage applied to the signal electrode for that period to be held till the end of the scanning period of the one line. If the insulating resistance of the signal electrode to another unit is insufficient, the voltage applied to the signal electrode capacitor till the end of the scanning period is released so that the voltage applied to the TFT of a pixel unit drops. As a result, each pixel connected with that signal electrode has an uneven luminance for each signal electrode because the applied voltage is always low. In order to prevent this, the TFT element at the output step of the driver should be held on till the end of the scanning period of one line so that an electric current may be supplied to an extent corresponding to the discharge of the voltage from the signal electrode.
Next, it is necessary to consider the problems of the ON characteristics of the TFT elements of the display unit and the output step. As the display is of a higher capacity, i.e., a larger display area and more scanning lines, the scanning periods of one line and one pixel become shorter. Since the electrostatic capacity per line becomes higher, on the contrary, a relatively higher electrostatic capacitive load has to be charged up for a short period for either a so-called "sequential dot scanning method", by which signal lines are sequentially scanned by one signal line for one scanning period, or a scanning method of sequentially scanning by a plurality of signal lines (the latter method will be called the "sequential block scanning method by making one block of a plurality of lines to be once scanned). The TFT element at the output step of the driver should also have a high mutual drain conductance gm. According to the aforementioned scanning methods, moreover, the ON voltage of the TFT elements of the display unit are so reduced that an insufficient voltage is applied to the liquid crystal and the contrast ratio of the display is reduced. This makes it necessary to enlarge the channel width W of the TFT elements to thereby increase the mutual conductance gm. As a result, the circuit area is increased, and the ratio occupied by the display electrode of the display unit is reduced together with the display characteristics. In order to avoid this, the so-called "sequential line scanning method", by which the TFT element of the display unit is turned on for the substantially whole address period of one scanning line with the signal voltage being applied, is desired as the driving method.
Next, the structure of a built-in driver or a driver at a signal side (or data voltage generating side) is required to have high-speed operations so that care should be taken with regard to the circuit design. If the number of the pixels of the display unit of a display is assumed to be expressed by N (i.e., the number of vertical pixels) .times.M (i.e., the number of horizontal pixels) and if the frequency for rewriting one frame (which will be called the "frame frequency") is denoted at f.sub.F (Hz), for example, the maximum frequency f.sub.max of a signal voltage inputted to the display is calculated by N.times.M.times.f.sub.F. With the pixel number of the display unit being N=400, M=640.times.3 (assuming the display of three colors R, G and B) and f.sub.F =60 Hz, for example, the maximum frequency f.sub.max takes such a very high value as is expressed by f.sub.max =46.08.times.106 Hz=46.08 MHz. Since the circuit operating within such frequency band is very difficult to be constructed of TFTs of amorphous or polycrystalline silicon, for example, it is necessary to improve the circuit structure or the signal applying method having characteristics matching the TFT elements. The above-specified example of the prior art is a circuit structure which has been devised to apply input data in parallel to thereby to drop the aforementioned maximum frequency f.sub.max with the number of the input data. However, the part for receiving the signals from the outside and the part for applying the input signals to the display unit are of the voltage distribution type resorting to the electrostatic capacity, in which the common TFT elements are used or in which the TFT elements are used as transfer gates. As a result, the example of the prior art requires the TFT elements of the input part to drive a high electrostatic capacitive load so that it is defectively difficult to respond to an input signal of high frequency.
In the aforementioned embodiment, moreover, the timing for applying or the circuit structure for generating the drive voltage such as scanning pulses for operating the TFT elements for processing the input data signals divides the selection period of one scanning line with the number of blocks, each of which is composed of a plurality of signal lines. Since the pulse width of the scanning pulses becomes smaller for a larger frame and the higher fineness, a circuit for generating the scanning pulses is required of high-speed operations.
The prior art thus far described has failed to efficiently process the high-speed input data of a built-in signal driver using TFTs to apply them to the display unit so that it has been troubled in its own operating speed and the display characteristics of the display unit.